1. Field of the Invention
The present invention relates generally to a rotational position sensor and, more particularly, to a rotational position sensor that permits the offset and gain of its output signals, provided by a magnetically sensitive component, to be mechanically adjusted and calibrated without the necessity of electronic calibration.
2. Description of the Prior Art
Many different types of rotational position sensors are known to those skilled in the art. In many applications where the rotational position of a rotatable member is monitored through the use of a magnetically sensitive component, one of two basic methods is employed. First, a magnetically sensitive component can be configured with a permanent magnet that provides a bias magnetic field. The bias field provided by the permanent magnet is imposed on the magnetically sensitive component and, when a ferromagnetic object attached to a rotatable shaft passes through a predetermined detection zone, the magnetic field is distorted and the component of the field imposed on the magnetically sensitive component changes. An output signal is provided to indicate this change which is representative of the movement of the rotatable member.
A second method for determining the angular position of a rotatable member is to rigidly attach a magnet to the rotatable member and then measure the effect that the rotating magnet has on a stationary magnetically sensitive component, such as a Hall effect element or a magnetoresistive element. As the magnetic field rotates with the permanent magnet and with the rotatable member, the effective component of the magnetic field imposed on the stationary component changes and can be used to represent the angular position of the rotatable member.
Many different types of angular position sensors can be configured according to the two basic concepts described above. The sensor can employ one or more magnetically sensitive components and the components can either be Hall effect elements or magnetoresistive elements. In addition, many types of pole piece structures have been developed to direct the magnetic field in desirable paths.
One particularly effective angular position sensor is described in U.S. Pat. No. 5,164,668 which issued to Alfors on Nov. 17, 1992. The rotational position sensor described in U.S. Pat. No. 5,164,668 is provided with a decreased sensitivity to the position of the shaft on which a permanent magnet is attached. The angular position sensor is provided with first and second pole pieces that extend from regions that are proximate a rotatable magnet to regions that are proximate a magnetically sensitive device. The pole pieces provide defined magnetic paths of lower reluctance that confine the lines of flux extending between the rotatable magnet and the magnetically sensitive device. The placement of the rotatable magnet between first and third pole piece segments of the invention significantly reduces the sensitivity of the sensor to variations in position of the rotatable magnet and therefore increases the reliability of the measuring system. This reduced sensitivity inhibits the degradation of operational accuracy that could otherwise be caused by inaccuracies in the magnet's shaft position, large tolerances in the dimensions of the shaft diameter and the bearing diameter and variable location of the shaft because of excessive bearing wear.
In most angular position sensors known to those skilled in the art, an electronic calibration step is required if the angular position sensor is expected to achieve high accuracy in its measurement of the angular movement of a rotatable member. However, an electronic calibration step following the assembly of a rotational position sensor increases its costs and inhibits certain other manufacturing processes because of the fact that the electronic circuitry must remain exposed to permit calibration operations to be performed. Typically, these calibration processes comprise the trimming of resistors which are provided in conjunction with a circuit connected to the magnetically sensitive component. It would therefore be significantly beneficial if a means could be provided to allow the angular position sensor to be adjusted and calibrated mechanically without the need for electronic calibration.